BACKGROUND OF THE INVENTION

This invention relates to the preparation of shell molds and to compositions useful in the manufacture of refractory coatings. In particular, this invention relates to aqueous suspensions of refractory materials.

The investment cast process has proven to be of high value in the preparation of castings of numerous items, particularly small and intricate castings. It produces castings with high dimensional accuracy, superior surface and excellent shape.

In this process, an expendable pattern is made of wax or a plastic material. These expendable patterns are given repeated coatings of refractory materials in order to prepare a shell mold. Normally the coatings are obtained by applying a slurry of the refractory material in hydrolyzed ethyl silicate or colloidal silica, which can be catalyzed with an a ine or dried to form a hard shell. Once the shell has formed, the pattern is removed by use of heat or solvent resulting in the desired mold.

The strength of the shell mold apparently is caused by the silane bond formed by the silica contained in the ethyl silicate or colloidal silica in the refractory slurry used to coat the expendable pattern and to form the shell mold. The number of layers of refractory forming the coating determines the strength of the shell. Thin shells require the support of sand when the metal is poured into them during the casting of the required parts, whereas thicker shells produced from five to ten coatings normally do not require this support being strong enough to support the poured metal.

While the product of the investment casting process is excellent, the cost of the process has limited its use. This high cost is, at least, partially caused by the need to use the described slurry of the refractory material which need be prepared at the site of the casting operation.

It is, therefore, an object of the present invention to develop a procedure for preparing shell molds that does not use a slurry of the refractory materials in hydrolyzed ethyl silicate or colloidal silica.

It is another object of the present invention to devise compositions containing refractory materials that can be transported distances and stored so as to eliminate the need to prepare the composition at the site of the casting.

Another object is to lower the cost of performing the investment casting process.

Other objects will become apparent from the ensuing description.

SUMMARY OF THE INVENTION

It has now been found that storable aqueous suspensions of refractory materials containing an inorganic binder selected from the group consisting of aluminum monophosphate, sodium silicate and potassium silicate can be prepared as a premix refractory composition useful for the preparation of shell molds.

These aqueous refractory suspensions can be shipped, ready for use at the casting site and only require mixing before being applied to the expendable pattern in the preparation of shell molds.

DETAILED DESCRIPTION The aqueous refractory suspensions of the present invention contain as their essential components

water, refractory materials and an inorganic binder selected from the group consisting of aluminum phosphate, sodium silicate and potassium silicate. In addition, other minor components, such as wetting agents, suspending agents, viscosity modifiers, preservatives, organic binders and other like materials known to the art for the preparation of such suspensions can be present in a total amount that should not exceed five weight percent of the suspension, preferably less than two percent by weight of the suspension. These materials improve the quality of the suspension by modifying the viscosity and surface tension of the water. Among materials that can be used are fluorinated surfactants such as the ZONYL brand products, alkylphenols such as nonylphenol, sodium lauryl sulfate, al ylarylsodium sulfonate, polyethylene glycol ether of primary and secondary alcohols, alkyl phenol ethoxylate, cellulose materials such as methocell, hydroxyethylcelluloεe, carboxymethylcellulose and the like. The advantage of using a small amount of an organic binder is that while the inorganic binder of the present aqueous refractory suspension will not burn out when heating the shell mold and will cause solubilization of the prior coating when the multiple coatings are being applied, the organic or secondary binder will not be solubilized by subsequent coatings, but it will burn out during the heating phase. In order to obtain a strong shell mold it is necessary to apply several coats of the refractory suspension. If the coat being applied solubilizes the prior coat, then the new coat will not remain firm and it will be difficult to build up a sufficient thickness of refractory material. Organic binders include polyvinylacetate emulsions, acrylic emulsions and polyvinyl alcohol. The amount of water in the suspensions is that necessary to form a suspension of the refractory material and binder.

Normally the water will constitute between about 20 and about 35 weight percent of the suspension.

The refractory material to be used in the present emulsions can be selected from the broad classification known to the art. Included are quartz, silica, zirconia, zircon, mullite, aluminoεilicates, alumina fused silica and others. The refractory material constitutes the major amount of the present aqueous dispersion, i.e. from about 50 to about 75 weight percent, preferably about 60 to about 70 weight percent.

It has been found in preparing and testing the compositions of the present compositions that it is desirable that the refractory be present in more than one shape. This is particularly useful in the preparation of castings of intricate design where it imparts improved dimensional strength to the shell mold. Thus, the presence of from about one to about ten weight percent of the refractory material, in the form of fibres, in the present composition is a preferred option. Fibres normally have a length of about from less than about 0.25 to about 0.50 inches. Other shapes are possible.

The inorganic"binder component of the present refractory suspension constitutes about five to about ten weight percent of the suspension, i.e., it is normally present in an amount of from about 10 to about 25 weight percent of the refractory material. The exact amount of each component may vary with the identity of the specific components without changing the usefulness of this suspension.

The following Examples illustrate refractory compositions within the scope of the present invention:

EXAMPLE 2 Component Amount (Pounds) Aluminum silicate mineral 665 Aluminum silicate fibres 9 Sodium silicate 80 Suspending agent 1 Attapulgite clay 2 Bactericide 0.8 Defoaming Agent 0.6 Water 191.6

EXAMPLE 3

EXAMPLE 4

Aluminum silicate mineral 340

Aluminum silicate spheres 200

Aluminum silicate fibres 9

Defoaming agent 4

Aluminum phosphate 75

Polyvinyl acetate emulsion 9

Water 191.6

EXAMPLE 7

The refractory material used in these Examples was aluminum silicate; however, it can be replaced with other refractory materials without other modification. In addition, the following products were used in preparing the products of the Examples:

KYANITE Aluminum Silicate Mineral ZEOSPHERES Aluminum Silicate Spheres VFS 110 Aluminum Silicate Fibres

KELZAN Suspending Agent ATTAGEL 50 Attapulgite Clay DOWICIDE A Bactericide TROY-D126 Defoaming Agent

Shell molds were prepared by applying several coatings of the above-identified suspensions to polystyrene patterns. The patterns were removed by heating and castings were prepared by pouring aluminum, iron and steel into the mold. All of the castings obtained from molds prepared from the above-identified mold were satisfactory meeting the requirements of shape and strength.

In preparing shells by the application of the present refractory suspension, it is necessary to form multiple coatings of the refractory material on the pattern. In doing so, it is necessary to dry each coating prior to the application of the next coating. Drying can be readily accomplished by air or other means. The number of coats of refractory to be applied to the pattern is a function of the strength required for the shell mold. Three or four coatings of refractory on an expanded polystyrene pattern will create a thickness of 1/8"-1/4". At that time, the polystyrene can be removed by the use of solvent or by heating the shell in a furnace at a temperature of about 600°F to about 2,000°F. Additional heating of the shell at these temperatures will remove any organic or other material that is deposited in the pores of the shell. In addition, the heating will increase the strength of the shell. Heating to 700°F will impart sufficient strength to the shell to allow the casting of steel.

As previously indicated, the present refractory suspensions constitute a sufficient cost savings over the use of slurries of ethyl silicate and colloidal silicate. The fact that the shell molds obtained by use of the present aqueous suspensions have

sufficient strength after heating to 700°F represents a significant savings in energy costs since the slurry technique produces shell molds which need be heated to about 1,200°F to 1,800°F so as to develop adequate strength for the pouring of castings.